Why Is The Oceans Salty: Science & Myths

by Jay | Posted on July 25th, 2022

For this post, let us unravel the story behind how our seas and oceans came to be and uncover the science behind their inherent saltiness. We will learn why sea water is not drinkable. We will also get fresh perspectives from different cultures on their theories about the saltiness of the earth’s seas.

Prepare yourself for a ride through the stories of our seas. 

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The Natural Water Cycle: The Secret of the Salt

Let us look back at how the world began. Back when everything was still fresh out of a gas explosion, it took millions of years for the Earth to recover from the massive release of energy. Then, finally, our beloved blue planet began to cool itself down, and solid land slowly appeared around that time.

The Earth’s atmosphere has not fully formed yet, and the liquid form of water has not existed yet. However, as the planet’s temperature gradually decreased over time, the gases began to undergo condensation, transitioning into the liquid state, forming water, and falling to the Earth’s surface.

But when the Earth’s crust was still not cool enough, the liquid water would evaporate back into the atmosphere and back again. Eventually, the planet’s surface became cool enough for the water to remain in its liquid form. And that was when the large bodies of water began forming. 

Scientists think that the first oceans of the Earth were less salty than the ones we have now. But, according to their speculations, that was still the beginning of the water cycle, so the water that circulated from the earth’s surface to the atmosphere and back again did not have much-dissolved content yet.

Safe to say, the water back then was still pure. However, as time went by and the water cycle continued, the water that evaporates into the atmosphere gathers more and more carbon dioxide, forming carbonic acid molecules – a carbonate of mild acidity. 

As it returns to the ground, the water becomes more acidic, allowing it to become more and more prone to dissolving substances from the ground that it touches, which are most likely sodium and chlorine ions.

The water cycle repeats this process over and over until the large bodies of water settle into their final forms, carrying large amounts of solid content that the water has gathered from years worth of dissolving, evaporating, and condensing. 

The constantly flowing bodies of water like the rivers, streams and lakes supplied with fresh water are not salty because their constant movement does not allow these ions to settle long enough to form sediment particles.

The same cannot be said for the oceans and seas, which are generally static bodies of water. They are large, but they are almost immobile and are not constantly flowing, allowing the ions to settle and form into large sediment particles. 

The salt-forming ions like sodium and calcium begin their chemical reaction in these bodies of water, thus, making our oceans and seas saltier and saltier over time. This claim is supported by surveys and analyses conducted as far back as the 1950s, and the data collected since then proved that the Earth’s oceans are becoming more and more saline. 

There are two places from which the salt can originate: rock deposits on land and cracks on the ocean floor.

The slightly acidic rainwater is one of the causes of the erosion of rocks. And since rocks have mineral deposits, they usually end up in rivers and streams, and the seas and oceans are their last destination. So while these minerals are often consumed by marine life, the ones not consumed will accumulate over time. 

The second source comes directly from underneath the ocean itself. There are small openings and cracks on the ocean floor.

And when the heat from the Earth’s core in the form of magma reacts with the water in the ocean through those cracks, the reaction triggers a chain of different chemical reactions that causes the formation of magnesium and sulfate ions as well as absorbing metallic elements like iron, zinc, and copper.

Volcanic eruptions in the ocean also release abundant minerals directly into the water.

In geology, there is also a thing called salt domes. These geological formations originate from large masses of mineral deposits from former bodies of water that have dried up due to evaporation. For example, several salt domes can be found in the Gulf of Mexico.

Sodium and chloride are the two most common icons in seawater. 85% of the dissolved content in the Earth’s oceans is made up of these two main ions. 10% consists of magnesium and sulfate ions. The remaining percentage is other ions of varying concentrations.

The salt saturation also depends on many environmental factors like temperature, and, of course, the stages of the water cycle are evaporation and precipitation. Finally, suppose we’re talking about the geographical location.

In that case, it’s safe to say that there are lower salinity levels for the waters near the equator and at the end poles and increases towards the mid-latitude areas. Normal salinity measures at 35 parts per thousand or 3.5% of the weight of seawater. 

Variables that Affect Salinity

Salinity is a term used interchangeably with total salt content. This includes all salt-forming elements in seawater, such as chloride, sodium, and magnesium. Salinity is expressed in parts per thousand (o/oo). So if the normal salinity is translated, there are 35 pounds of salt in every 1,000 pounds of seawater. 

Salinity can be affected by other environmental phenomena like the melting of ice caps, the inbound flow of water from freshwater sources, rain, evaporation, snow, wind, the tide, and the ocean current that all cause water movement. 

A more common approach to salinity analysis is determining the salt content in a 1,000 g equivalent water weight. Normal levels fall between 34 to 36 parts per thousand. 

Salt levels can also affect water properties in ways like:

• Increasing the density of the water with the increase of salt content. 
• In addition, Saltwater has a lower freezing point than fresh water. 

High salinity areas

Our oceans do not receive the same rainfall, but evaporation still occurs. When water evaporates, the salt deposits remain, and their concentration in the seawater increases over time, leading to the higher density of that particular body of water. As you can see in this graphic representation, the areas around the Atlantic with heavy winds but less rainfall have high saline levels.

As mentioned above, normal ocean salinity is between 34 to 36 parts per thousand. Saline levels are regulated by evaporation and inbound freshwater and rain. In terms of evaporation, the Mediterranean Sea in Europe has more than 38 ppt of salinity because it experiences more evaporation than freshwater and rainwater influx. 

Low salinity areas

There are parts of the Earth’s oceans that receive an abundance of rainfall. Since rainwater is mainly freshwater, it reduces the salinity of the ocean water by increasing the amount of water. As a result, saline concentration decreases along with the density of the ocean water. The part of seawater near the land and freshwater sources also tends to have lower salt levels.

The ocean near Antarctica and the Arctic have low salinity levels because icebergs provide freshwater as they melt. As a result, salt levels in Antarctica fall below 34 parts per thousand and even more so in the Arctic, where it falls below 30 in particular areas. 

We know that temperature and salt content affects the density of water. When it reaches its freezing point at 0°C, the hydrogen molecules rearrange to form a rigid, open-lattice structure that is more spread out than the liquid form of water, making the ice less dense and causing it to float. 

A good example is the Baltic Sea, surrounded by northern Europe and Scandinavia, with salinity below 10 parts per thousand. The main reason for this is the many freshwater sources surrounding the sea.

Changes in salinity

Even the slightest difference in salinity levels can affect water’s density and behavior. Add that with temperature changes, and you will significantly impact the overall density of a certain water sample.

If seawater has a low temperature, it becomes so dense that it moves downwards into the ocean, where it settles just above ocean basins, flowing in slow currents. 

Are salinity levels in all seawater equal?

To put it simply, no. There are always variances in the salt content levels of our planet’s different seas and oceans. Therefore, the most accurate way of determining salinity is through sample analysis.

And this can be expressed in terms of grams of salt per kilogram equivalent of water. Take, for example, the Atlantic and Pacific Oceans. Both of them have 35% salinity. In other words, 35 grams of salt are required for every kilogram of water. 

But again, this does not apply to all seas and oceans on the planet. For example, the Baltic Sea has a 10% salinity level or only 10 grams of salt for 1 kilogram of its water. Therefore, it is also referred to as brackish water. 

The salt crust in the Dead Sea

As much as there are extremely low salinity levels, there are also extremely low ones. With a salt content percentage that shoots up to 337% based on recent findings, the Dead Sea’s salinity continues to increase because of the lack of rainfall but continuous evaporation.

It has no connection to any fresh water source, which is why its waters have a huge salt concentration. The Dead Sea has lost all its attachments to other bodies of water, making it impossible to receive a freshwater supply. 

Why is salt water undrinkable?

There is an enormous, unlimited water supply in the world, but it is unwise to drink directly from the source. In our normal human physiology, we need both water and salt in our body, so you would think that having a readily available source of both would be a real convenience.

However, that is not the case. Ever heard of the quote, “too much of something is a bad thing”? That applies to seawater being an electrolyte replenisher substitute in the human body. Even just a small amount of seawater already contains too much salt than the body can handle.

We need salt in our body as it is essential to regulating many physiological functions and maintaining pH levels. Still, seawater is not a reliable source for that. It is also extremely unsanitary due to other possible foreign contaminants in seawater. Even our urine is less salty than seawater. 

Hypernatremia

You might think that it’s strange why drinking seawater could cause harm when it contains substances that we need to survive. This is because the body has a natural way of resolving imbalances in our system.

An excess of salt will cause the body to eliminate what it doesn’t need. One way to do that is through sweat or urination. This means that to release excess salt, the body has to release water along with it. So the more salt you take, the more water it would require to eliminate the excess, which can result in dehydration.

Hypernatremia is a condition with too much sodium in your blood circulation. Common symptoms are nausea and bloated feeling of thirst. In severe cases, the person could experience muscle spasms, confusion, and internal hemorrhaging in some parts of the head, and the worst part is that it can even cause death.

Oceans vs. Rivers: Why the former is saltier

The larger body of water is where the water from smaller bodies ends up. So the water from the rivers flows into the oceans. And that is just one reason contributing to the ocean’s salinity. 

Scientists presume that the Earth’s oceans may not have been this full of salt during the early days. However, when the water cycle began and continued, rainfall caused the erosion of rock and mineral deposits which then led to these minerals being washed away into the ocean. 

Rivers are generally connected to the seas and oceans, so their flow will always end up there, including the minerals from rocks. And since the rain also dilutes the freshwater, there is little to no salt left in those bodies of water. 

Looking at the numbers, 225 million tons of dissolved solids and 513 million tons of rock and mineral sediments a year are being transported to the ocean from rivers and streams in the United States alone.

Now, if we look at it from a global perspective, that sums up to about 4 billion tons of dissolved salts being fed into our oceans yearly.

However, most of these salt deposits become sediments on the ocean floor, so they may not significantly affect the ocean’s overall salinity. So chances are, there is an equal proportion of the amount of salt that goes in and out of the Earth’s oceans.

Comparisons between seawater and river water 

1. The two elements in table salt (sodium and chloride) make up around 85% of the dissolved solids in the ocean and contribute the most to its salinity. Still, they are only at around 16% of the total salt content in river water. 
2. Calcium is mainly the mineral deposit that gets washed stream from the river into the ocean, but there are far more chloride ions than calcium ions by about 46 times. 
3. Silica, being the most abundant mineral on Earth, is abundant in river water but, surprisingly, not in seawater.
4. Other ions, such as calcium and bicarbonate, are also found in large amounts in river water but not much in the ocean. These ions are only below 2% of the ocean’s total solid content.

Marine life is also a large factor in the variations of the content in ocean water. There are also a lot of living organisms in the ocean with unique biochemistry that consume the content in the water for their survival.

Sea creatures with exoskeletons like mollusks utilize the calcium deposits in the water to build their shells, as well as crustaceans like crabs, shrimps, and lobsters. Corals are also made up of limestone (calcium carbonate) that has accumulated from other corals and sea creatures for several years. 

Planktons, often called marine drifters, are tiny microorganisms that play a huge role in the marine ecosystem. They are sources of food for other sea creatures. They are at the bottom of the ocean food chain. Some plankton species like diatoms rely on the silica content of the ocean to build their skeletal makeup. 

There are also sea creatures that secrete some traces of elements that make up the composition of ocean water. For example, lobsters have zinc, copper, and cobalt in their tissues. Snails have lead. Sea cucumber absorbs vanadium. Sea sponges and some seaweeds absorb iodine content from the water.

The marine ecosystem contributes a lot to the composition of the ocean waters. But certain elements like sodium are bound to remain untouched in the water since no known marine life consumes the sodium content in the ocean. 

Other factors like solubility and chemical reactions between elements and water affect salinity levels. For example, if a substance is soluble in water, it will most likely have a high concentration in the water.

Otherwise, if it’s not entirely soluble, some traces will precipitate and become sediments on the ocean floor.

Calcium does not completely dissolve in water and therefore reacts with other salts to become calcium carbonate and sediment on the ocean floor. Manganese and phosphorite are also examples of water-insoluble substances that become ocean floor deposits.

Stories and Legends from the Ancient Times

Before we could examine our natural resources scientifically, people had the same questions but had no means to obtain factual information. All they had was their imagination and creativity to develop their theories on how and why the ocean is the way it is. 

The Denmark Coffee Mill

This story is about an orphan boy named Hans who lived with his grandmother. Right before his grandmother died, she left him her most prized possession in the form of a coffee mill.

Whatever Hans needed, the mill would grind. All he had to do was ask. Then, the mill would grind the things that he required, and there would be a special command that he had to say to stop the mill from grinding. 

The mill made his life a lot easier. It became his means of survival. As he got older, Hans yearned to travel the world. He became a sailor. But other sailors mistreated him, giving him only small amounts of food.

This did not bother him because the mill provided him with everything he could ever need. However, when the other sailors discovered his secret, they grew envious of him. So the sailors killed Hans and took the mill for themselves. 

Because the sailors were filled with greed and hunger for wealth, they asked the mill to grind salt, and they would sell it. But they failed to know the special command to stop the mill from grinding.

So the mill kept creating salt ceaselessly to the point that it sank the whole ship. Legend has it that up to this day; the mill is still grinding salt because it has not been given the special command to stop. 

The ocean and the salt maker in the Philippines

Pedro is a very poor man who owed his friends and neighbors many debts to survive. But unfortunately, he could not repay what he owed even though he kept promising that he would. So he went to another island on his trusty, old boat to make new acquaintances. 

He ended up on an isolated island and stumbled upon an old lamp. He initially planned on selling it, but as he was cleaning it, a genie emerged from the lamp and struck him a deal that would bring him out of poverty.

The genie would make salt for him to sell, provided that he would share his earnings with family and friends. But, of course, he also had to make the trip to the island every time he got his salt supply, and the lamp should never leave the island. 

The man accepted the deal, carried as much salt as possible in his old boat, and made a fortune. He was able to rise out of his poverty; he was able to provide for his family. But he grew unsatisfied and arrogant. He violated a part of the deal he made with the genie. He took the lamp from the island, planning to bring it home. 

Suddenly, salt began to gush out of the lamp in large amounts. The boat sank, along with Pedro and the lamp that is said to be still creating salt today. 

Fortunately for today, due to the advancement in our ways, we can now gather accurate information and determine the science behind the natural occurrences our world undergoes.

We won’t have to concoct fictional stories to come up with explanations that are mostly too far-fetched. As much as coffee mills and lamps go, these are works of overly creative imaginations. 

The inherent saltiness of the Earth’s oceans results from many natural phenomena – the water cycle, land erosion, and marine biology, among others. And as long as nature is at work, there is no human intervention that can stop it. Our oceans and seas will remain so until the end of days because that is how they should be. 

Jay

Self assessed Germaphobe, specializing in everything water, water filters, health and nutrition. Diagnosed with Type 2 Diabetes, I've acquired immense amount of knowledge when it comes to natural, biology, and everything about human anatomy.